EMB120 AND TWIN CESSNA HAD LESS THAN COMFORTABLE SEPARATION IN ZDV CLASS E AIRSPACE.

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THE ZDV CTLR GAVE US A DSCNT CLRNC TO 16000 FT WITH A RESTR TO DSND AT A RATE OF AT LEAST 1500 FPM. SHE SUBSEQUENTLY ASKED US IF WE WERE PROCEEDING 'DIRECT LARKS.' I REPLIED THAT WE WERE NOT; AS WE HAD BEEN CLRED DIRECT POWDER INTXN DURING THE COURSE OF OUR INITIAL CONTACT WITH ATC. AT APPROX 3 MI FROM THE POWDER INTXN; WE WERE INSTRUCTED TO CONTACT DEN APCH. THE APCH CTLR'S FIRST INSTRUCTIONS ON INITIAL CONTACT WERE TO 'DSND IMMEDIATELY TO ONE FIVE THOUSAND.' I WAS THE PF; AND AS THE INSTRUCTIONS WERE ISSUED; I HIT THE TRIM SWITCH WHICH ALLOWED ME TO BOTH DISCONNECT THE AUTOPLT AS WELL AS COMPLY WITH THE ATC INSTRUCTIONS. I GLANCED AT THE TCASII WHICH WAS STILL ON THE 20 MI RANGE AND SAW A TARGET AT APPROX OUR 8 O'CLOCK POS. THE TARGET WAS A SOLID CIRCLE; BUT I DON'T RECALL HEARING AN AURAL TCASII ALERT. BECAUSE WE WERE IN VMC; I GLANCED UP AND SAW A WHITE TWIN CESSNA WITH TIP TANKS PASS OVERHEAD BY WHAT WAS AT LEAST 300 OR 400 FT. I STATED TO THE APCH CTLR THAT IT APPEARED A LOSS OF SEPARATION HAD OCCURRED. I INFORMED HIM THAT I WOULD BE CALLING THE AREA MGR TO DISCUSS THE INCIDENT.